The present invention relates to the field of system networks, and, more particularly, to methods and systems for converting data transmitted across a network from a first representation to a second representation.
Hypertext Transfer Protocol (HTTP) is a network protocol used to deliver most resources on the so-called “World Wide Web.” A resource is a chunk of information that can be identified by a universal resource locator (URL). The most common kind of resource is a file, but a resource can also be a dynamically-generated query result, the output of a CGI script, as well as various other types of data structures.
When a HTTP client opens a connection and sends a request message to an HTTP server, the server usually returns a response containing the resource that was requested. The format of the request and the response are generally similar. Both kinds of messages typically include an initial line, zero or more header lines, a blank line, and an optional message body. The initial line, called the status line, provides the HTTP version, a computer readable status code and a human readable phrase describing the status code. The header lines provide information about the data sent in the message body. If a HTTP response includes the resource that was requested in the response's message body, there are usually header lines in the response that describe the body.
A client may be connected to a server directly or through one or more intermediaries. For example, a client may choose to use one or more HTTP proxy servers as intermediaries. A proxy server is a relay computer system that is located somewhere on a network path between a server and client computers. A proxy server receives requests from clients, and forwards those requests to the intended servers. Any response from the servers also passes back through the proxy server then to clients. A proxy server thus performs functions of both a client and a server. Often, proxy servers have high bandwidth connections to servers and low bandwidth connections to clients. Thus, part of a proxy server's duties may be to convert a high-resolution response from the server to a low-resolution response for the client so that the response can be more efficiently transferred to an displayed at the client. A client may explicitly specify to the proxy server its preference to receive high or low resolution responses. For details, see the explanation provided in the article entitled “Adapting to Network and Client Variation Using Active Proxies: Lessons and Perspectives,” by Armando Fox, Steven D. Gribble, Yatin Chawathe, and Eric A. Brewer, Special issue of IEEE Personal Communications on Adaption, August 1998, which is hereby incorporated by reference as background information. If a client is directly connected to servers (i.e., without using proxy servers), the client may convert a response from a high resolution representation to a low resolution representation in order to display the response on a low resolution display.
Clients vary along many axes, including screen size, color depth, effective bandwidth, processing power, and ability to handle specific data encodings, e.g., GIF, PostScript, or MPEG. A typical server response may include text, graphic images, audio data, video data or other multimedia contents. When converting a response, the proxy server or client has many options. For example, if a response includes graphic images, the size and resolution of the image may be reduced. In addition, the number of colors and the spatial frequencies of the image may also be reduced. If a response includes a plurality of graphic images, the order in which the images are sent to a client can be rearranged. If a response includes audio data, the audio data can be compressed or sampled at a lower rate. If a response includes video data, the frame rate may be reduced. Furthermore, large pages can be partitioned into multiple connected sub-pages.
Some existing systems attempt to convert server responses from one representation to another representation that is more suitable for transmission to and displaying at client computers. Representation conversion is sometimes referred to as “transcoding” or “distillation.” It is known in the art that transcoding is distinct from “compression.” Transcoding is typically lossy while compression may or may not be lossy. Lossy conversions often cause data degradation and the decrease in the size of the representation.
In one type of existing system, where a client communicates with an origin server (the original source of the content) via a proxy server, the proxy server has a limited set of page conversion procedures that it applies to convert any received responses. Since the proxy server has a finite set of page conversion procedures for all incoming web pages, these page conversion procedures may be inadequate for conversion of some responses. Examples of such systems include the TranSend service of the GloMop (Global Mobile Computing by Proxy) project and the MeGA (the Media Gateway Architecture).
In another type of existing system, servers may provide instance-specific instructions to proxy servers or clients. An example of this type of existing system is described in the following article which is hereby incorporated by reference as background information: A Survey of Active Network Research, David L. Tennenhouse, Jonathan M. Smith, W. David Sincoskie, David J. Wetherall, and Gary J. Minden; IEEE Communications Magazine, Vol. 35, No. 1, pp 80-86. January 1997. Although servers in the systems described in this article do provide some instance-specific instructions, these instructions are primarily provided at the data packet level, which is the lowest level of the network. Namely, these instructions are applicable per data packet (which constitutes portions of a response) transferred across the network, but do not apply to the requested response as a whole. In addition, the instructions are implemented by switches or routers within the network; thus, utilizing this type of system can exacerbate transmission congestion across the network.
Another type of existing system is described in the following article, which is hereby incorporated by reference as background information: On Active Networking and Congestion, Technical Report GIT-CC-96-02, Samrat Bhattacharjee, Kenneth L. Calvert and Ellen W. Zegura; College of Computing, Georgia Tech. 1996. The system described in this referenced article preferentially drops low-priority data packets in a data stream. Some data packets are dropped when a network becomes congested. One major disadvantage of this type of systems is the likelihood that valuable data packets may be dropped. Priority policies are implemented at the data packet level by switches or routers; thus, this type of systems increases the work load of switches and routers and could increase network congestion.
Another type of existing system converts web contents to formats displayable on low-resolution screens. An example of this type of system is WebTV. WebTV allows users to browse web sites by using a standard TV screen as the display. Since television screens typically have lower resolution than most computer monitors (also called computer screens), WebTV converts various types of web content to formats suitable for display on television screens. Typically, this type of system uses a generic conversion procedure to convert all web pages to “television pages,” without regard to the content of the page(s) being converted. Thus, the same conversion procedure is used to convert photographs, texts, graphic charts of data, and so on. Thus, the format conversion performed by such system is neither instance-specific nor content-specific.
Another type of existing system, described in U.S. patent application Ser. No. 09/048,652, entitled “Method for Dynamically Adapting Web Content and Presentation to Network Conditions,” filed by Jeffrey Mogul and Lawrence Brakno on Mar. 16, 1998, which issued as U.S. Pat. No. 6,243,761 on Jun. 5, 2001. This application is hereby incorporated by reference as background information. In the system as described in the referenced application, the origin server has the burden of adapting content for a particular client. There are several disadvantages to this system. First, in many information distribution systems the origin server is not set up to receive and retain information about each particular client's display capabilities, often because doing so would be expensive and impractical. Second, the origin server may not have enough resources to perform all the necessary representation conversions for a large number of clients having different display capabilities.
In summary, there remains a need for an information distribution system that is configured to handle a variety of representation conversions that take into account both the display capabilities of client devices as well as the content of the images or other data being converted.